feat(web): centralise Vietnamese price formatting across all pages

Create a single `currency.ts` utility with `formatPrice`, `formatVND`,
`formatPricePerM2`, and `parseVND` to replace 9+ duplicated inline
formatters. This fixes inconsistent decimal handling (1.5M was truncated
to "1 triệu") and standardises price/m² display. Integrated across
property cards, listing detail, dashboard, analytics, payments, pricing,
and admin moderation pages with 19 new unit tests.

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/web/app/[locale]/(public)/pricing/page.tsx

@@ -13,6 +13,7 @@ import {
   CardTitle,
 } from '@/components/ui/card';
 import { Link } from '@/i18n/navigation';
+import { formatVND } from '@/lib/currency';
 import { usePlans } from '@/lib/hooks/use-subscription';
 import type { PlanDto } from '@/lib/subscription-api';
 import { cn } from '@/lib/utils';
@@ -134,16 +135,6 @@ const FALLBACK_PLANS: PlanDto[] = [
 // Helpers
 // ---------------------------------------------------------------------------
 
-function formatVND(amount: string | number): string {
-  const num = typeof amount === 'string' ? Number(amount) : amount;
-  if (num === 0) return 'Miễn phí';
-  if (num >= 1_000_000) {
-    const millions = num / 1_000_000;
-    return `${millions % 1 === 0 ? millions.toFixed(0) : millions.toFixed(1)} triệu đ`;
-  }
-  return num.toLocaleString('vi-VN') + ' đ';
-}
-
 // Feature labels mapped for the comparison table
 const FEATURE_LABELS: Record<string, string> = {
   maxListings: 'Tin đăng',